Indwelling probes have been proposed for measuring various physiological parameters. For example, sensors on the tips of the probe have been designed to measure the partial pressures of carbon dioxide, oxygen, and pH (frequently referred to as the "blood gases"). In use, the sensors of such probes are positioned at or slightly beyond the distal end of a conventional hypodermic needle inserted within the arterial or venous bloodstream. A limitation to the accuracy of these sensors is the formation of clotting material such as thrombi or blood clots on or near the probe tips. Such clots can form within ten minutes to several hours following in vivo insertion of the probe. These clots in turn metabolize and create a microenvironment around the sensors that affect the accuracy of the sensors. In practice, sensors surrounded by such clots measure lower pH (more acidic) and higher pCO.sub.2 ("partial pressure of CO.sub.2 ") than when such clots are not present.
Several methods have been used in the art to overcome such problems. Probes have been coated with antithrombogenic materials, such as heparin, in an attempt to impair clot formation. Because heparin works best in relatively slow velocity conditions, it is ineffective in the high-velocity arterial system. Furthermore, heparin coatings tend to wear off with blood exposure, thereby becoming less effective over time.
Another technique to minimize and control clot formation is to deliver a supply of liquid or flushant at a very slow rate that is sufficient to flush the lumen or bore of the catheter to prevent clots from forming within the needle bore. One difficulty with this method is that a probe generally will lie tangent to one side of the catheter's exit orifice. As a result, the flushant will be diverted to the opposite side of the exit orifice, thereby minimizing the effectiveness of the flush in the region of the probe-catheter tangency. Consequently, clots of large size can eventually form around the probe and compromise the reliability of the sensor. Increasing the concentration of the flushant to prevent or dissolve these clots causes unacceptable errors in sensor performance. Furthermore, probes projecting beyond the end of the catheter will not receive the benefit of the flushant because the force of the blood flow will wash the flushant away. Thus, there is a need for a flushing system that can minimize and control clot formation without impairing sensor performance.